1. Field of the Invention
The present invention relates in general to an electrophotographic printing system and more particularly to an improved electrophotographic printing system that ensures that the center of a calibration patch is evaluated.
2. Description of the Related Art
An electrophotographic printing system needs to continually regulate the toning density of the imaging subsystem by periodically reading image density and adjusting various imaging parameters to maintain the desired image density. The imaging subsystem can be adjusted by attempting to print a uniformly toned patch (or target) onto the sheet transport mechanism and then reading the density of the printed patch with a densitometer. In order to accurately read the patch, the patch position on the transport mechanism must be accurately tracked from the point of it being printed onto the transport mechanism to the point of it being read by the densitometer sensor. The imaging subsystem can determine the appropriate place to read the patch by estimating the distance between these two points but, due to mechanical differences between systems, this distance must be accurately determined through calibration. One way of performing this is to have the print engine automatically locate the position of the process control density patch using the densitometer by taking periodic density reading samples while the sheet transport mechanism moves the patch through the sensor.
One such method that addresses this problem is disclosed in U.S. Pat. No. 5,953,555 dated Sep. 14, 1999, which is incorporated herein by reference. The key point of the design in U.S. Pat. No. 5,953,555 is that the area of the process control patch is calculated from the sampled density patch waveform and the patch positional centerline is calculated and compared to a predetermined positional centerline. The difference/delta between the patch centerline and the predetermined centerline is used as an automatic timing or position adjustment for subsequent patch readings used for toning density regulation. This design requires a solid uniform patch to be printed. If the uniformity is inconsistent, the calculated centerline of the patch may not be accurate.
The invention described below overcomes the problems of the conventional system by providing a system that locates a trigger point or offset within the calibration patch and limits the evaluation to areas in the center section of the patch.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional printing system, the present invention has been devised, and it is an object of the present invention, to provide a structure and method for an adjustable automatic process control density patch location detection.
In order to attain the object suggested above, there is provided, according to one aspect of the invention, a method of testing a process control patch in an electrophotographic printer. The invention first observes/creates a beginning transport medium lead edge location, which establishes a reference point on a transportation device. Next, the invention tracks the position of the transport mechanism within the printer by starting a counter triggered by the medium lead edge sensor or by a created virtual medium lead edge signal that is internally machine generated. The invention then prints the process control patch on the transportation device. The invention identifies positions of the transportation device and process control patch locations once the process control patch passes through a densitometer sensor. The invention measures the width of the patch against a predetermined patch width and tests the measured width for accuracy. The invention establishes a patch testing area determined by a calculated patch trigger point and user configurable sample window parameters. The difference between the leading edge and the trailing edge represents the width of the process control patch. The patch testing area is smaller than the width of the process control patch. The invention later evaluates the print image density of the patch testing area using the sensor during normal print operations and other calibration routines.
The invention tracks the position of the lead edge location or reference point through the use of a counter that is started upon the generation of the lead edge signal. The patch testing area omits testing (or sampling) points of the process control patch that are outside of the area defined by the beginning patch trigger point and the user configurable sample window parameters. The size of the patch testing area and number of samples taken within the patch testing area are user-adjustable. The invention records the beginning patch trigger point.
In another embodiment, the invention samples a process control patch in an electrophotographic printer by first printing a process control patch on a print medium transportation device, defining a patch testing area within the process control patch (such that the patch testing area is smaller than the process control patch), and determining printing density in the patch testing area.
The invention defines the patch testing area by identifying the process control patch on the print medium transportation device and monitoring movement of the print medium transportation device. The process of determining the printing density is restricted to the patch testing area based on the monitoring of the movement of the print medium transportation device. The patch testing area is bounded by the user configurable sample window parameters. The patch testing area is determined by sensing the media leading edge location on the print medium transportation device and monitoring the movement of the print medium transportation device to locate the patch testing area at a predetermined distance from the media leading edge. The density within the patch testing area is determined by testing using densitometers, to determine density of printing.
The invention also provides a system for sampling a process control patch in an electrophotographic printer. The system includes a print medium transportation device and a printing element adjacent the print medium transportation device. The printing element prints the process control patch on the print medium transportation device. The system further includes a counting mechanism attached to the print medium transportation device. The counting mechanism defines a patch testing area within process control patch such that the patch testing area is smaller than the process control patch. Also, the system includes a densitometer adjacent the print medium transportation device. The densitometer determines the printing density in the patch testing area.
The counter defines the patch testing area and monitors movement of the print medium transportation device. In normal operation, the densitometer locates the patch testing area based upon a count of the counter. In calibration mode, the densitometer is used to locate the process control patch and the position of where the process control patch was found on the print medium transportation device is recorded, based on the count of the counter when the process control patch was identified by the densitometer.
The system also includes a print medium leading edge sensor that senses the initial media edge (real or virtual) on the print medium transportation device for triggering the start of a process control patch reading cycle. The counter locates the patch testing area at a predetermined distance from the print medium leading edge, and is used together with the user configurable sample window parameters for sampling the print density.
As mentioned above, in calibration mode, the densitometer, in conjunction with output from the counter, determines printing density to identify a position of leading and trailing edges of the process control patch on the print medium transportation device. The counter is used to calculate a patch trigger point for the patch testing area based on a user definable trigger point. The user definable trigger point identifies the distance for a beginning patch trigger point of the patch testing area from the leading edge of the process control patch. Preferably, the beginning patch trigger point and user configurable sample window provide a patch test area that is centered within the process control density patch.